It does not depend on the vendor but on the particular class implementing the Set interface: some sets are unmodifiable, so you cannot add anything to them. The Javadoc specifies any usage limitation for each implementing class. For example a Set returned by Collections.unmodifiableSet() is, obviously, unmodifiable.

Perhaps during the upgrade of Java you have also upgraded some dependency and the new version of that dependency returns an unmodifiable Set. Check the Javadoc of the method that returns that Set.

You'll probably have to modify your code and copy the content of that Set in a new modifiable instance, for example:

Set<YourClassGoesHere> newSet = new HashSet<>(oldSet);

The set will give much better performance (O(n) vs O(n^2) for the list), and that's normal because set membership (the contains operation) is the very purpose of a set.

Contains for a HashSet is O(1) compared to O(n) for a list, therefore you should never use a list if you often need to run contains.